Respiration in health and at extremes
The respiratory system adapts to exercise, altitude, and diving, and its link to blood pH makes it vital for chemical balance.
Breathing and blood pH
Carbon dioxide forms carbonic acid in blood, so breathing controls pH. Hypoventilation (breathing too little) raises carbon dioxide and lowers pH: respiratory acidosis. Hyperventilation (breathing too much, as in panic) lowers carbon dioxide and raises pH: respiratory alkalosis, causing tingling and dizziness.
Exercise
During exercise, breathing can rise from about 6 litres per minute at rest to over 100 litres per minute. The increase starts immediately, driven by the brain and signals from moving joints, then is fine-tuned by chemoreceptors. Training improves the muscles' and heart's use of oxygen more than it improves lung capacity.
High altitude
At high altitude the air has the same 21% oxygen but lower pressure, so oxygen partial pressure falls. The body responds by breathing faster, and over days to weeks by making more red blood cells through EPO. Altitude sickness, with headache, nausea, and in severe cases fluid in the lungs or brain, can occur if people ascend too quickly.
Diving
Underwater, pressure rises by about one atmosphere every 10 metres. Divers breathing compressed air absorb extra nitrogen into their tissues; rising too fast lets it form bubbles, causing decompression sickness ('the bends'). Breath-hold divers rely on the diving reflex, which slows the heart and redirects blood to vital organs.
Defences of the respiratory system
Besides mucus and cilia, the lungs have alveolar macrophages that engulf particles and microbes that reach the alveoli. Coughing and sneezing are reflexes that clear irritants. Vaccines against influenza, pneumococcus, and COVID-19 reduce serious respiratory infections.
Smoking and the lungs
Tobacco smoke damages cilia, inflames airways, destroys alveolar walls, and contains dozens of carcinogens. Smoking causes most cases of lung cancer and COPD. Lung function decline slows after quitting, and the risk of lung cancer falls substantially over the following years.
Sleep apnoea
In obstructive sleep apnoea, the upper airway repeatedly collapses during sleep, stopping breathing for seconds at a time and waking the person briefly. It causes daytime sleepiness and raises the risk of high blood pressure. Treatment includes weight loss and continuous positive airway pressure (CPAP) machines.
Worked example
A person having a panic attack breathes very rapidly and feels tingling in their fingers. They are blowing off carbon dioxide, so blood pH rises (respiratory alkalosis). Slowing their breathing lets carbon dioxide build back up, restoring pH and relieving the symptoms.
Key takeaways
Breathing regulates blood pH through carbon dioxide: too little breathing causes acidosis, too much alkalosis. Exercise multiplies ventilation. Altitude lowers oxygen partial pressure, prompting faster breathing and more red cells. Divers risk decompression sickness. Macrophages, coughs, and vaccines defend the lungs. Smoking causes most lung cancer and COPD.
Flashcard
High CO2, low pH
Check your understanding
Why might a person breathing into a paper bag feel better during hyperventilation?